Vermouth, both Dry and Sweet are fortified wines that will go bad very quickly and should both be refrigerated. 2 months out will definately have some effect on the wine, but I don't think there is any safety issue with drinking it, especially considering 99% of the bars out there have the same bottles of vermouth sitting out night after night after night.

You will definitely have an impact in flavor, although it may not be too noticeable right now.

I much prefer to buy smaller bottles of vermouth and keep them in the fridge so I have a higher turnover.

In my own experience, fortified wines like vermouth do not go bad quickly; they keep well for months after opening. I find that exposure to light is bad for them -- sweet vermouths turn from garnet to an unattractive brownish --, but they certainly don't spoil within days the way unfortified wines do. I don't go through tons of vermouth, so some of my bottles are over a year old, but I don't find the deterioration of flavor over time that fafner cites.

I have been doing a lot of research on this lately for a series of articles I'm working on. Vermouth should be kept in the fridge and out of the light. Also that you only buy it from stores with fast turnover where it has only been on the shelves for a few days or weeks. It should be used within six months of bottling, max. up to around 8 months. Before the one year point it will have started to go off in taste. This is for unopened bottles. Opened bottles should be used well within 1month for best flavor. Vermouth doesn't have enough fortification to prevent it from going off quickly. I did a taste test between unopened bottles that were new vs. a few months old and you could easily tell the difference. Also between just opened and in the fridge for two months and the difference was very noticeable. The older they were, the more oxidized and off tasting they became.

Appearance is the same, completely clear, and a swirl produces the same results, basically no ring and a lot of tiny clinging droplets. The new vermouth has a crisp, sweet, floral and herbal nose with hints of light thyme, curry, a touch of allspice, some lavender, honeysuckle and muscat wine. Taste is light, crisp and very clean. The best description would be a somewhat light, white dessert wine, sweetened, more herbal, with a noticeably boozier kick to the flavor profile. Finish is short, clean, and mildly dry with lingering sweet honeysuckle, pear, fresh thyme, and general floral notes. Greats stuff that would be excellent served chilled over ice, or on the rocks with a bit of soda and twist of orange or lemon.

The six month old has a noticeably weaker nose, with some slightly stale and musty odors and perhaps just a slight metallic note suggesting a hint of oxidation. That said, there is a strong underlying similarity in terms of the base wine and the overall herbal impression. Furthermore, there are no “off” odors suggesting excessive oxidation screaming out from the glass, and it’s not an unpleasant aroma; really, it’s basically just weaker, less dynamic, slightly stale, and as such, a tad less good. The taste is somewhat similar - there are no off notes of vinegar suggesting excessive oxidation nor anything truly repulsive. The biggest difference is that there is no real depth of flavor, the sweetness is stronger and more generalized, and the whole thing feels a bit flabby. The finish is not as dry, the alcohol notes rougher, and the sweetness stronger and less complex and refined. All that said, it’s not bad, and IMO it could certainly be used in a cocktail without any real damage. Would the fresher Dolin be a step up? Sure, certainly for some cocktails. I made a round of martinis (2.5 oz of Boodles to .75 oz vermouth) a few weeks back using the old Dolin and they were fine. I do think the more pronounced and unrefined sweetness made a slight difference, but it was a real measure of degrees, and overall, they were still damn fine martinis that got very good reviews from my guest whose favorite drink, it’s worth noting, is a martini.

I've also done a similar comparison for Lillet Blanc and found about the same thing.

I've had no problem keeping them unrefrigerated after opening, though I mostly buy sweet. As mentioned, I probably would want to use it up within 6 months, but I obviously don't have the highly refined palate of others who find an "off" taste.

I wouldn't throw out the stock you have even if you decide to start refrigerating. Taste them if you're not sure, but my guess is you certainly should have no problem using them as mixers or even drinking straight.

Vermouth is wine, so it will turn to vinegar if left out. Any non-distilled liquor requires refrigeration after opening. According the experts at Noilly Pratt, an opened bottle of vermouth will last about 3 months if properly refrigerated.

I'm sure the manufacturers would be thrilled if they can convince people to buy a new bottle every 3 months. Taste your bottle yourself and make up your own mind. I've had bottles that have lasted for years stored in a dark cabinet. Sure, their flavor profile may have changed, but they still tasted fine. Definitely not vinegar!